Smallpox, once the scourge of millions, was in 1979 the first infectious disease to be declared successfully eradicated. This followed a worldwide campaign of vaccination by the World Health Organisation over the previous decade, and was made feasible by the fact that humans are the only reservoir for the virus. Vaccination is a preventative strategy that aims to stimulate the host immune system, by exposing it to the infectious agent in question in an inactivated or incomplete form. There are four main classes of virus vaccines:
• Attenuated (='weakened') vaccines contain 'live' viruses, but ones whose pathogenic-ity has been greatly reduced. The aim is to mimic an infection in order to stimulate an immune response, but without bringing about the disease itself. A famous example of this type of vaccine is the polio vaccine developed by Albert Sabin in the 1960s. The cowpox virus used by Edward Jenner in his pioneering vaccination work in the late 18th century was a naturally occurring attenuated version of the smallpox virus.
• Inactivated vaccines contain viruses which have been exposed to a denaturing agent such as formalin. This has the effect of rendering them non-infectious, while at the same time retaining their ability to stimulate an immune response. Vaccines directed against influenza are of this type.
• Subunit vaccines depend on the stimulation of an immune response by just a part of the virus. Since the complete virus is not introduced, there is no chance of infection, so vaccines of this type have the attraction of being very safe. Subunit vaccines are often made using recombinant DNA technology (Chapter 12); the first example to be approved for human use was the hepatitis B vaccine, which consists of part of the protein coat of the virus produced in specially engineered yeast cells.
• DNA vaccines are also the product of modern molecular biology techniques. DNA coding for virus antigens is directly injected into the host, where it is expressed and triggers a response by the immune system. Vaccines of this type have not so far been approved for use in humans.
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